1. Field of the Invention
The present invention relates to a signal transmission structure, more particularly to a signal transmission structure adapted to a signal transmission media for improving impedance mismatch of a signal transmission route.
2. Description of the Related Art
Because electronic technologies have been advanced, a variety of electronic products with different functions are gradually involved into our work and life. Electronic products usually have a motherboard that includes a circuit board and many electronic components. The electronic components are assembled on the circuit board, and are electrically connected thereto by internal routes in the circuit board for serving the functions for which the electronic products are designed, wherein these electronic components further comprises integrated circuit (IC) components. In order to efficiently protect IC components due to their fragile structure and electrically connect the IC components to external means, package technologies are used to achieve the functions mentioned above.
For IC components with a high pin count, ball grid array (BGA) technology has been widely used in the chip package field because it can provide a high pin count, better heat dissipation and good electrical performance. In realistic application, IC dies are electrically connected to an IC carrier through many conductive traces or bumps by wire bonding (WB) technology or flip chip (FC) bonding technology. Then the IC carrier is electrically and structurally connected to a large printed circuit board through a plurality of conductive balls by BGA technology. Therefore, two interfaces, two components or two terminals are separately formed on the IC carrier and the printed circuit board can transmit signals to each other through these conductive balls.
Referring to FIGS. 1A and 1B, FIG. 1A illustrates a top view of a signal transmission structure of the prior art, and FIG. 1B illustrates a cross sectional view of the signal transmission structure of FIG. 1A, which is applied to a signal transmission media. A signal transmission structure 100 (FIG. 1A) is adapted to be used in a signal transmission media 10 (FIG. 1B), such as a printed circuit board. The signal transmission structure 100 includes a reference plane 110, a bonding pad 120, a conductive trace 130 and a conductive ball 140. The reference plane 110 can be, for example, a ground plane or a power plane, which is embedded within the signal transmission media 10. The bonding pad 120 and the conductive trace 130 are formed by a circuit pattern 10a (FIG. 1B) on the surface of the signal transmission media 10. Moreover, the bonding pad 120 and the conductive trace 130 are electrically isolated from the reference plane 110 by a dielectric layer 10b (FIG. 1B); therefore, the bonding pad 120 and the conductive trace 130 are non-coplanar with the reference plane 110. The conductive ball 140 is located on a side of the bonding pad 120, which is away from the reference plane 110. In addition, the top of the conductive ball 140 is connected to a bonding pad 12a of another signal transmission media 12, such as an IC carrier, and then signals can be transmitted through a conductive trace 12b as shown in FIG. 1B.
Referring to FIGS. 1A and 1B, when signals go through the conductive trace 130, the bonding pad 120, the conductive ball 140, the bonding pad 12a and the conductive trace 12b, due to the geometric shape of the conductive ball 140, a non-continuous low impedance characteristic exits between the conductive ball 140 and the reference plane 110, and results in impedance mismatch at the signal route near to the conductive ball 140. Therefore, when signals go through the signal route near to the conductive ball 140, reflection of signals exists thereat, and leads to the problem that signals cannot be completely delivered from a signal terminal through the conductive ball 140 to another signal terminal. In addition, by the increase of the operational frequency of signals, a return loss of signals will reduce and an insertion loss of signals will increase while signals go through the conductive ball 140. Accordingly, impedance mismatch at the signal route near to the conductive ball 140 will be worse. That has impact for transmission of high frequency signals.